In anatomy, lymphatic vessels or lymph vessels are thin walled, valved
structures that carry lymph. As part of the lymphatic system, lymph vessels are
complementary to the cardiovascular system. Lymph vessels are lined by
endothelial cells, and have a thin layer of smooth muscles, and adventitia that
bind the lymph vessels to the surrounding tissue. Lymph vessels are
devoted to propulsion of the lymph from the lymph capillaries, which are mainly
concerned with absorption of interstitial fluid from the tissues.
Lymph capillaries are slightly larger than their counterpart capillaries of
the vascular system. Lymph vessels that carry lymph to a lymph node are called
the afferent lymph vessel, and one that carries it from a lymph node is called
the efferent lymph vessel, from where the lymph may travel to another lymph
node, may be returned to a vein, or may travel to a larger lymph duct. Lymph
ducts drain the lymph into one of the subclavian veins and thus return it to
general circulation. Generally, lymph flows away from the
tissues to lymph nodes and eventually to either the right lymphatic duct or the
largest lymph vessel in the body, the thoracic duct. These vessels drain into
the right and left subclavian veins respectively.
Structure The general structure of lymphatics is
based on that of blood vessels. There is an inner lining of single flattened
epithelial cells composed of a type of epithelium that is called endothelium,
and the cells are called endothelial cells. This layer functions to
mechanically transport fluid and since the basement membrane on which it rests
is discontinuous; it leaks easily. The next layer is that of smooth muscles
that are arranged in a circular fashion around the endothelium, which by
shortening or relaxing alter the diameter of the lumen. The outermost
layer is the adventitia that consists of fibrous tissue. The general structure
described here is seen only in larger lymphatics; smaller lymphatics have
fewer layers. The smallest vessels lack both the muscular layer and the outer
adventitia. As they proceed forward and in their course are joined by other
capillaries, they grow larger and first take on an adventitia, and then smooth
muscles. The lymphatic conducting system broadly
consists of two types of channels—the initial lymphatics, the prelymphatics or
lymph capillaries that specialize in collection of the lymph from the ISF,
and the larger lymph vessels that propel the lymph forward.
Unlike the cardiovascular system, the lymphatic system is not closed and has
no central pump. Lymph movement occurs despite low pressure due to peristalsis,
valves, and compression during contraction of adjacent skeletal muscle
and arterial pulsation. = Lymph capillaries =
The lymphatic circulation begins with blind ending highly permeable
superficial lymph capillaries, formed by endothelial cells with button-like
junctions between them that allow fluid to pass through them when the
interstitial pressure is sufficiently high. These button-like junctions
consist of protein filaments like platelet endothelial cell adhesion
molecule-1 or. A valve system in place here prevents the absorbed lymph from
leaking back into the ISF. There is another system of semilunar valves that
prevents back-flow of lymph along the lumen of the vessel. Lymph capillaries
have many interconnections between them and form a very fine network.
Rhythmic contraction of the vessel walls through movements may also help draw
fluid into the smallest lymphatic vessels, capillaries. If tissue fluid
builds up the tissue will swell; this is called edema. As the circular path
through the body's system continues, the fluid is then transported to
progressively larger lymphatic vessels culminating in the right lymphatic duct
and the thoracic duct; both ducts drain into the circulatory system at the right
and left subclavian veins. The system collaborates with white blood cells in
lymph nodes to protect the body from being infected by cancer cells, fungi,
viruses or bacteria. This is known as a secondary circulatory system.
= Lymph vessels = The lymph capillaries drain the lymph to
larger contractile lymphatics, which have valves as well as smooth muscle
walls. These are called the collecting lymphatics. As the collecting lymph
vessel accumulates lymph from more and more lymph capillaries in its course, it
becomes larger and is called the afferent lymph vessel as it enters a
lymph node. Here the lymph percolates through the lymph node tissue and is
removed by the efferent lymph vessel. An efferent lymph vessel may directly drain
into one of the lymph ducts, or may empty into another lymph node as its
afferent lymph vessel. Both the lymph ducts return the lymph to the blood
stream by emptying into the subclavian veins
The functional unit of a lymph vessel is known as a lymphangion, which is the
segment between two valves. Since it is contractile, depending upon the ratio of
its length to its radius, it can act either like a contractile chamber
propelling the fluid ahead, or as a resistance vessel tending to stop the
lymph in its place. Function
Lymph vessels act as reservoirs for plasma and other substances including
cells that have leaked from the vascular system and transport lymph fluid back
from the tissues to the circulatory system. Without functioning lymph
vessels, lymph cannot be effectively drained and edema typically results.
= Afferent vessels = The afferent lymph vessels enter at all
parts of the periphery of the lymph node, and after branching and forming a
dense plexus in the substance of the capsule, open into the lymph sinuses of
the cortical part. It carries unfiltered lymph into the node. In doing this they
lose all their coats except their endothelial lining, which is continuous
with a layer of similar cells lining the lymph paths.
Afferent lymphatic vessels are only found in lymph nodes. This is in
contrast to efferent lymphatic vessel which are also found in the thymus and
spleen. = Efferent vessels =
The efferent lymphatic vessel commences from the lymph sinuses of the medullary
portion of the lymph nodes and leave the lymph nodes either to veins or greater
nodes. It carries filtered lymph out of the node.
Efferent lymphatic vessels are also found in the thymus and spleen. This is
in contrast to afferent lymphatic vessels, which are found only in lymph
nodes. Clinical significance
Lymphangiomatosis is a disease involving multiple cysts or lesions formed from
lymphatic vessels. See also
Meningeal lymphatic vessels Additional images
References External links
Lymphatic Vessels at the US National Library of Medicine Medical Subject
Headings Efferent lymph vessel - BioWeb at
University of Wisconsin System Physiology: 3s3ch9_5 - Essentials of
Human Physiology